It is well known in the art of semiconductor manufacturing that depth of focus (“DOF”) is an important issue. Fukuda (Hitachi Central Research Labs) proposed a method to increase DOF using FLEX (Focus Latitude Enhanced eXposure) in 1989, wherein the exposure is performed using two stage focal positions. This is discussed in U.S. Pat. No. 4,869,999, entitled METHOD OF FORMING PATTERN AND PROJECTION ALIGNER FOR CARRYING OUT THE SAME, issued to Fukuda, et al on Sep. 26, 1989 (“Fukuda I”), where the specification also notes that:
It has been found by the inventors' investigation that the effective focal depth of an exposure optical system can be increased by overlapping a plurality of light beams having image points at different positions on an optical axis, and thus the image of a mask pattern can be formed accurately in a region between the top and the bottom of the topography of a substrate surface. The term “image point” indicates a point on the conjugate plane of the mask pattern with respect to the exposure optical system. Accordingly, when an exposure operation for exposing a substrate coated with a resist layer to exposure light through a mask is performed a plurality of times at different positional relations in the direction of the optical axis between the resist layer and the image plane of a mask pattern, or when exposure operations at the different positional relations are simultaneously performed, the image of the mask pattern can be accurately formed not only at the top and the bottom of the topography of a substrate surface but also at an intermediate position between the top and the bottom of the topography. Thus, a fine pattern can be formed accurately all over the topography. (Col. 3, lines 33–54, emphasis added)
Fukuda I also states:
Furthermore, in the present embodiment, the image plane of a mask pattern was formed at two different positions in (or over) the substrate by displacing the substrate in the direction of an optical axis. Alternatively, the image plane of the mask pattern may be formed at different positions by moving a reticle having a mask pattern in the direction of the optical axis, by introducing a transparent material different in refractive index from air into an exposure optical system, by changing the atmospheric pressure in the whole or a portion of the exposure optical system, by using a lens having a multiple focal point, by overlapping light beams from a plurality of exposure optical systems which form the image plane of a mask pattern in different planes, or by using different wavelengths or a continuous wavelength in the same exposure optical system. (Col. 6, lines 37–53, emphasis added)
It has also been proposed, e.g., in systems sold, e.g., by Nikon that a stepper allow continuous stage motion between two focal planes.
In U.S. Pat. No. 4,937,619, entitled PROJECTION ALIGNER AND EXPOSURE METHOD, issued to Fukuda, et al. on Jun. 26, 1990 (“Fukuda II”), there is proposed a system in which separate laser beams are generated and optically combined to produce a single beam with a plurality of different wavelengths arriving at the reticle in the lithography tool at the same time. Fukuda II also notes:
FIG. 5 is a configuration diagram of a third embodiment of the present invention. The embodiment shown in FIG. 5 comprises a reflecting mirror 31, an etalon 32, an excimer laser gas cavity 33, an output mirror 34, a mirror 35, an etalon angle control circuit 36, a laser oscillation control circuit 37, an exposure wavelength control circuit 38, an illumination optical system 14, a reticle 15, a projection lens 16, a substrate stage 17, and various elements required for the projection aligner.
The etalon 32 narrows the bandwidth of the laser beam oscillated by the excimer laser resonator composed of a reflecting mirror 31, excimer laser gas cavity 33, and an output mirror 34, and changes the central wavelength of light narrowed in bandwidth by adjusting the angle of the etalon 32 minutely. The wavelength control circuit 38 sends a command to the etalon angle control circuit 36 to set the angle of the etalon at a predetermined value, and sends a command to the laser oscillation control circuit 37 to cause laser oscillation with a predetermined number of exposure pulses for the etalon angle. The exposure wavelength control circuit 38 is capable of changing the set angle of the etalon 32 during the exposure of one exposure region located on the substrate by using the above described function and is capable of performing projection exposure by using light having a plurality of different wavelengths. Since the projection lens 16 focuses the pattern on the reticle 15 onto a different position on an identical optical axis with respect to each of the above-described plurality of wavelengths, it is possible to perform the focus latitude enhancement exposure by using the present projection aligner.
Instead of being disposed between the reflecting mirror 31 and the laser resonator 33 as shown in FIG. 5, the etalon 32 and the wavelength control means may be disposed between the output mirror 34 and the laser gas cavity 33, or between the output mirror 34 and the illumination optical system 14, for example. Further, the above described line narrowing and wavelength alteration are not restricted to the method of changing the angle of the etalon.
The present embodiment is economically advantageous because only one excimer laser is used. In addition, lowering of laser output caused by bandwidth narrowing can be limited to a small value because the bandwidth-narrowing device is disposed between the reflecting mirror and the output mirror.
By using the present projection aligner, it was confirmed that the depth of focus of fine patterns increased in the same way as the first embodiment.
In U.S. Pat. No. 5,303,002, entitled METHOD AND APPARATUS FOR ENHANCING THE FOCUS LATITUDE IN LITHOGRAPHY, issued to Yan on Apr. 12 1994, there is proposed also combining separately generated laser beams to obtain a single beam at the reticle with a plurality of wavelengths. Yan also proposes the generation of three output beams from a single laser system, but the embodiment proposed is not workable.
In the prior applications assigned to applicant's assignee referenced above “spectral engineering” has been proposed using, e.g., a wavelength and bandwidth tuning mechanism to produce an apparent spectrum over a series of pulses in a burst of pulses output by the laser system that effectively contains a plurality of discrete spectra. The '294 patent and '925 application suggest that:                A fast responding tuning mechanism is then used to adjust center wavelength of laser pulses in a burst of pulses to achieve an integrated spectrum for the burst of pulses approximating the desired laser spectrum. The laser beam bandwidth is controlled to produce an effective beam spectrum having at least two spectral peaks in order to produce improved pattern resolution in photo resist film. . . . In a preferred embodiment, a wavelength tuning mirror is dithered at dither rates of more than 500 dithers per second in phase with the repetition rate of the laser. . . . In another embodiment, the maximum displacement was matched on a one-to-one basis with the laser pulses in order to produce a desired average spectrum with two peaks for a series of laser pulses. Other preferred embodiments utilize three separate wavelength tuning positions producing a spectrum with three separate peaks. (Abstract)        
The Specifications of the '294 patent and/or the '925 Published Application referenced above also describe FIGS. 2A, 2B, 2C1–C3, 2D1–D3, 2E, 2F, 2G1–G3, 2H1–H3, 10A, 10B, 10C, 10D, 10E, 10F, 10G, 10H and 10I the descriptions of which are hereby incorporated by reference. The Specification also specifically notes, referring to the paragraphs in the '925 application as published:                A lithography technique, called FLEX (short for, “focus latitude enhancement exposure”) has been shown (through simulation and experiment) to improve the depth of focus by utilizing multiple exposure passes of the same field with different focus settings. This technique is also commonly referred to as focus drilling, since the physical thickness of the photoresist film is exposed in multiple passes at incremental focus settings. The image in photoresist is formed by the composite of the multiple exposure passes.                    Several difficulties result from this FLEX process with both step and scan as well as step and repeat exposure implementations. Multiple pass exposure results in additional overlay (image placement) errors and image blurring. This has further implications on process latitude, focus repeatability as well as wafer throughput since multiple exposures require multiple imaging passes.                        
A broad laser spectrum can offer some DOF improvement at the expense of contrast. FLEX, using the Z-axis focus control of the scanner stage, and the embodiments proposed by Fukuda II and Yan, using separate sources of laser output light at different center wavelengths which are thereafter optically combined, can produce two images focused in separate image planes, separated temporally or not. Using light arriving at the same time at the wafer through the scanner optics can, e.g., re-create the original double focal plane concept of FLEX by taking advantage of the longitudinal chromatic aberration of the PO lens (250–450 nm/pm)
The magnitude of the bandwidth effect will depend on many factors, such as, lens NA, lens chromatic aberration, feature size and type.
The effects of bandwidth may be similar to other causes of variation such as defocus, lens aberrations; partial coherence. The shorter the wavelength the more severe the consequences, e.g., 193 nm lenses have generally higher chromatic aberration than 248 nm lenses and a higher sensitivity of other aberrations to wavelength. It has also been determined that isolated and dense lines are affected in different ways, with some benefit for isolated lines but little improvement for dense lines. The benefits of the improved DOF appear to be optimum for certain types of imaging, e.g., contact hole imaging.
Certain other lithography parameters may also be affected, e.g., dose may have to be slightly higher for this RELAX technique. Optical proximity effects, linearity and mask error factor, line end shortening and perhaps other parameters will likely be affected. Effects on lens performance, e.g., distortion/displacements and aberrations may need to be determined.
U.S. Pat. No. 5,835,512, issued to Wada et al. on Nov. 10, 1998, entitled WAVELENGTH SELECTING METHOD IN WAVELENGTH TUNABLE LASER AND WAVELENGTH SELECTABLE LASER OSCILLATOR IN WAVELENGTH TUNABLE LASER, discloses an acousto-optical wavelength selection apparatus for selecting the wavelength at which a laser oscillator resonates, the disclosure of which is hereby incorporated by reference. U.S. Pat. No. 6,016,216, issued to Chang on Jan. 18, 2000, entitled POLARIZATION-INDEPENDENT ACOUSTO-OPTIC TUNABLE FILTER, discloses an acousto-optical filter utilized for wavelength division multiplexing, the disclosure of which is hereby incorporated by reference. U.S. Pat. No. 6,404,536, issued to Lean et al. on Jun. 11, 2002, entitled POLARIZATION INDEPENDENT TUNABLE ACOUSTO-OPTICAL FILTER AND THE METHOD OF THE SAME discloses an acousto-optical filter utilized to pass one beam without impact from the filter and another that is tuned by the filter.
It is well known that there is an interaction of light in certain materials that is a function of some external stimulation on the material, e.g., a acoustic stimulation (acousto-optic), an electrical stimulation (electro-optic) or even magnetic (magneto-optic). For example in an acousto-optic device, e.g., a non-linear crystal there occurs an effect associated with the interaction of light with sound, e.g., in the change of the diffraction of light by the acoustically perturbed medium, as is discussed in, e.g., P. Kerkoc, et al, “Molecular crystals for applications in acousto-optics” Phys. A: Math. Gen. 32 No 20 (21 May 1999), the disclosure of which is hereby incorporated by reference. As noted in Keroc when an acoustic wave propagates in a medium, there is an associated strain field. The change in the index of refraction that results from the strain is known as the photo-elastic effect. Since the strain field induced by the acoustic wave is a periodic function of position, the perturbation of the index of refraction of the medium is also periodic, leading to coupling between the modulating strain field and the optical wave. In this way, any transparent material can, e.g., be made to act as an optical phase grating. The photo-elastic effect, unlike the linear electro-optic effect, occurs in all states of matter and, in particular, in crystalline material belonging to all symmetry classes. Devices based on the acousto-optical effect operate at only a few volts as compared with the kilovolt or so required in an electro-optical modulator. However, the range of useful acousto-optical materials is limited, either because of their low figure of merit or as a consequence of the strong attenuation of ultrasound. At present there are, e.g., TeO2, LiNbO3, GaAs, GaP and PbMoO4 materials used as acousto-optical devices and some highly polar molecular crystals, e.g., 3-methyl-4-nitropyridine-1-oxide crystal (POM) used for acousto-optical properties. Other crystals may prove effective for applications according to aspects of the present invention, including, e.g., single crystals MgF2, KDP, SiO2, PWO, CaMoO4, Ge, Si etc. Additionally, nonlinear optical organic crystals, e.g., 2-α-(methylbenzylamino)-5-nitropyridine (MBANP), which can show both strong electro-optic and piezo-electric responses may be useful for acousto-optical applications as suggested in the present application.
The effect can be utilized in a Raman-Nath cell or a Bragg cell.
The design and fabrication of bulk acousto-optic modulators (temporal modulation) and beam deflectors (spatial modulation) are described, along with, e.g., device parameters that can be obtained systematically for given specifications are discussed in E. Young et al., “Acousto-optics, deflectors, light modulation, modulators, bandwidth, design analysis, fabrication, impedance matching, light beams, wave interaction,” IEEE, Proceedings, Vol. 69, January 1981, p. 54–64, the disclosure of which is hereby incorporated by reference. Young discusses modulation bandwidth, throughput efficiency and number of resolvable elements, and excitation of the devices, e.g., at up to a few hundred megahertz, and the effects. Young also discusses acoustic transducer response becoming sensitive to, e.g., intermediate metal layers between the piezoelectric transducer and the acousto-optic interaction medium. Also discussed therein are, e.g., criteria for material selection based on performance requirements and propagation loss. Practical considerations for the fabrication of high performance devices and specific device parameters are also discussed.
It is known in the art of high power gas discharge excimer (ArF or KrF) or molecular fluorine laser systems to select a center wavelength and narrow the bandwidth around the center wavelength by changing the angle of incidence of the inter-cavity light impinging on a grating line narrowing element. For Example as illustrated partly schematically in FIG. 5A, corresponding to FIG. 8 of U.S. Pat. No. 6,532,247, the disclosure of which is hereby incorporated by reference, and also corresponding generally to FIGS. 11A and 11B in U.S. Pat. No. 6,493,374, entitled SMART LASER WITH FAST DEFORMABLE GRATING, issued to Fomenkov, et al. on Dec. 10, 2992, the disclosure of which is also hereby incorporated by reference, a tuning mirror 14, comprising an essentially maximally reflecting mirror for a desired nominal center wavelength, e.g., around 192 for ArF excimer laser systems, may be mounted on a mirror mount 85 (14A in the '374 patent), as is further described in the '247 and '374 patent disclosures. It will be understood that the mirror 14 as shown in FIG. 5A is pivoted about a pivot axis (not shown) generally toward an opposite end of the longitudinal expanse of the mirror 14 from the end shown in FIG. 5A
Applicants in the present application propose improved methods and apparatus for carrying out RELAX, which enable the enhancement of DOF on the one hand and also enable more agile response to needs for modification of wavelength, e.g., pulse to pulse in a gas discharge laser system of the type described above.